Reference is made to FIG. 1 showing a conventional configuration for a touch screen system 10. The system 10 includes a touch panel 12 formed by a plurality of parallel drive lines 14 and a plurality of parallel sense lines 16. The drive lines 14 and sense lines 16 are typically formed of a transparent material (such as, for example, indium tin oxide ITO) so as to not obscure a visual display system (not shown) positioned underneath the panel 12. The drive lines 14 and sense lines 16 can, for example, each be formed of a plurality of series connected diamond shapes. The drive lines 14 extend across the panel 12 with a first orientation direction (for example, horizontal) and the sense lines extend across the panel 12 with a second orientation direction (for example, vertical) such that the lines 14 cross over the lines 16 (or vice versa). However, the plane containing the lines 14 and the plane containing the lines 16 are typically separated from each other by a layer of dielectric material. A sense capacitor 18 is accordingly formed at each location where the lines 14 and 16 cross.
A digital controller circuit 20 generates an alternating current (AC) drive signal (VTX), for example, in the form of a square wave, and sequentially applies that AC drive signal to the drive lines 14 through a driver circuit 22. The AC drive signal has a frequency fd that is, for example, in the range of 100-300 kHz and is typically at 200 kHz.
The digital controller circuit 20 is powered from a power supply voltage Vdd, with Vdd typically at 3.3V. The driver circuit 22, however, is powered from a power supply voltage Vddh, where Vddh>Vdd, with Vddh for example at 6V, 9V, 12V, 16V higher as needed. A charge pump circuit 24, powered from the power supply voltage Vdd, operates to boost the Vdd voltage to produce the Vddh voltage. An oscillator circuit 26 provides an AC signal 28 to the charge pump circuit 24 to control the boost switching operation of a flyback capacitor that generates the Vddh voltage. The AC signal 28 has a frequency fo that is, for example, in the range of 10-100 MHz and is typically at 48 MHz.
The driver circuit 22 includes a level shifting and buffering circuit to level shift the AC drive signal output from the digital controller circuit 20 from the Vdd voltage level to the Vddh voltage level to generate the level-shifted AC drive signal (Vdrive) for application to the drive lines 14.
A charge conversion circuit 30 such as a charge to voltage (C2V) converter circuit (or a charge to current (C2I) converter circuit) is coupled to the sense lines 16. The conversion circuit 30 senses the charge at each sense capacitor 18 and converts the sensed charge to an output signal (voltage or current) indicative of the sensed charge. The amount of charge at each sense capacitor 18 is a function of the AC drive signal, the capacitance between the drive line 14 and sense line 16 at the sense capacitor 18 and the influence of a touch capacitance contributed by the presence of an object (such as a finger or stylus) in proximity to the drive lines 14 and sense lines 16 of the panel 12. A processing circuit 32 receives the output voltages from the conversion circuit 30 for each sense capacitor 18. The output voltages are processed to determine the presence (touch and/or hover) of the object and the location of the object.